A person’s wisdom yields patience;
it is to one’s glory to overlook an offense. NIV Proverbs 19:11
It's going to happen. Some of your opponents will attack you personally, ad hominem style, or even just basic name calling. In one debate I had recently, it seemed like the guy on the other end had a whole list of pejorative terms that he was working into the argument, one by one, just to get a reaction.
Whatever you do, don't react in kind. Just let that garbage talk slide right off you, like you've got on Teflon tightie whities.
I've messed up a number of debates this way. My opponent would fire a shot, and I instinctively thought of further belittling counter, and shot right back. And it felt gooood! And then he fired another shot with his next counter, so I did the same. Wash. Rinse. Repeat. The problem was that the debate was unproductive. By the end, I could tell I had done little more that stroke my own ego, while having him think much less of me... not that he had admired me before!
I've come to realize that I need to stay focused on the goal. The goal is not to "win" in the theological equivalent to an "yo momma" insult contest. The goal is to change minds. Having your opponent think that you are a jerk is probably just reinforcing their same misunderstanding that they've already had about those without faith.
So when your opponent uses that kind of language, just ignore it. Just let it go. You'll actually feel better about it in the long run, and you may just have influence someone by changing their perception about atheists, if not about the debate topic.